Certain machine-generated noise in both residential and industrial environments, particularly continuous levels of low frequency noise, are disturbing to workers. The more pronounced types of incidental noise is linked to headaches, hearing impairments and other health-related conditions. This type of noise often may be conducted throughout a building structure to reach workers in locations well beyond the source. Noise generated and/or conducted through heating/cooling systems likewise can reach workers throughout a facility. In one approach to reducing the effect of machinery noise and vibrations, electromechanical devices are used as an actuator to actively generate predetermined forces or sounds near the noise source which cancel the noise energy. One such type of electromechanical device uses an electrostrictive material which in response to applied electric fields changes dimension or generates forces. Another material sometimes used in this application is piezoelectric.
Although electrostrictive materials have many advantages as actuators in active noise cancellation systems, their full utility has not been realized. One problem is that the electrostrictive structure, consisting of the material plus the driver electrodes, presents a substantial capacitive load to the control electronics. The result can be a low device operating efficiency.
Prior art actuator amplifier designs include rudimentary switching amplifiers and narrow band resonant circuits to recycle the energy and improve the electrical efficiency of the system. However, known switching amplifier designs generate substantial amounts of undesirable electrical noise. If this noise is allowed to operate on the electrostrictive structure, even more noise may be generated and propagated. Amplifier output signal filtering may be employed to reduce this effect, but the filtering in turn introduces signal phase shifts which must be compensated for by the control electronics. The net effect is a reduced control signal frequency span or bandwidth.
In addition, electrical noise generated by prior art switching designs tends to radiate and interfere with other sensitive electrical systems. Alleviating this further problem requires significant electrical shielding and power supply filtering at added costs.
In situations where the voltage/potential across the electrostrictive structure remains small, low electrical efficiency is tolerable. However, if large counter-forces are desired or if the volume of electrostrictive material is minimized to realize lower costs, large time varying voltages/potentials are required. In these cases, low electrical efficiency results in substantial power delivery and cooling difficulties, and increased energy costs. Particularly when a larger bandwidth or low electrical phase shift is required at large operating voltages the prior art offers no solutions.
Prior art designs of actuator amplifiers are also difficult to adapt to different voltage requirements. For instance, if a given switching amplifier design is to be used in both a high voltage application and a lower voltage application, either two designs are required with differing device voltage and current ratings; or one design incorporating the worst case use is required.